Over One Million Barriers on European Rivers

Katarina Samurović


Rivers of Europe have always been a central element of the continent’s life, economy, and culture. They are an integral part of European identity, as testified by the paintings of Claude Monet and Van Gogh, and classical compositions such as The Blue Danube – perpetually appreciated as a ravishing natural asset of Europe.

However, we sometimes forget that rivers are also living ecosystems and the zones of highest biological diversityWhen you consider their importance and prominence in the collective consciousness, it is disheartening that the actual state of the European rivers and their ability to support life have been much-neglected topics for a long time.

Instead of viewing them only as a mere resource, the European community has finally taken a systematic look at river health – the functional, living state of European rivers, as captured by the Water Framework Directive.

One of the main factors that endanger the ecological status of European rivers is the existence of in-stream barriers – as it turned out, an astounding number of them.

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What Is The Problem With River Barriers?

River barriers – from primitive weirs and to the advanced hydropower dams, are engineering achievements that have helped human inhabitants to control, redirect and use the river flow for their convenience. From flood control to milling to irrigation to hydroelectric power, it is no doubt that the in-stream barriers have had a unprecedented impact on modern society’s development.

Being seen as universally useful until very recently, we seldomly questioned whether all the existing dams and barriers should stay in place. They were perceived as a default, a necessity.

However, damming and barricading came at a high price for the rivers’ biodiversity, the ecosystem services, and local climates.

Even the simplest barricades slow down the natural flow of the rivers – that was their purpose, after all. However, this inevitably affects the stream’s physical and chemical properties. Furthermore, reservoirs and dams create hydrologic disturbances, trap nutrients (altering the water chemistry), and cause ecosystem fragmentation

There is a broader influence as well. For example, the creation of dam reservoirs increases local atmospheric humidity, which further affects the regional climate. And, despite being seen as a “clean” source of renewable energy, dam reservoirs and artificial lakes are a significant source of carbon dioxide, methane, and nitrous oxide – all potent greenhouse gasses.

The direct physical influence on aquatic life is also immense. Erratic downstream flow created by somehydropower plants has a devastating impact on river life since aquatic organisms don’t have time to adjust to abrupt changes in water levels, affecting their reproduction and survival.

Rivers’ fragmentation and low connectivity are among the most significant problems at the ecosystem level. 

Physical fragmentation of the habitat – breaking it into smaller, disjointed segments – disrupts usual migration routes, causes animal populations to become physically and genetically isolated, and dismantles the existing food network. In time, this leads to reproduction failures, population crashes, and local extinctions – especially when combined with other negative factors (such as overfishing in case of rivers).

The Infamous Demise of Danube’s Beluga Sturgeon

The most notable iconic European fish facing population collapse due to river fragmentation is the Beluga sturgeon (Huso huso– a living fossil and the largest freshwater fish in the world. As Larissa J. Graham and Brian R. Murphy noted in 2007:

Historically, beluga sturgeon were found in the Caspian Sea, Black Sea, Adriatic Sea, Sea of Azov, and all rivers within these watersheds. Today, naturally reproducing populations are only found in the Caspian Sea and Black Sea, and a few rivers in these areas.

A satellite image showing the flow of the Danube river into the Black Sea.
After flowing thousands of miles across Europe, the Danube empties in the Black Sea. Landsat 8 satellite image, August 13, 2018, NASA, public domain.

During its spawning migrations, the Beluga sturgeon used to swim further upstream than any other sturgeon species. The Danube was one of its destinations, and the surgeons traveled up this powerful river all the way to Central Europe. For example, it was documented that its bountiful flesh body provided sustenance for Austrians during the Middle ages.

The taste of the wealthy for the praised Beluga caviar has put this remarkable fish on the path of decline. However, in the 20th century, the river fragmentation has dealt a devastating blow to all sturgeons – especially the heavy-bodied and far-reaching Beluga sturgeon. 

A close up of a Beluga Sturgeon (Huso huso) swimming in an aquarium
Beluga Sturgeon (Huso huso). Photo: © mirecca/stock.adobe.com.

The surgeons upstream Danube migration was entirely interrupted by the Iron Gates dams project on the border of Romania and Serbia in the 1970s and 1980s; the two dams now constitute the largest such structures on Danube river. These hydropower giants were literal “gates” for sturgeons – the gates that never open – as they do lack fish passes or bypasses to assist fish migration. On top of it all, fishing continued with no regard for new, difficult circumstances.

According to Life for Danube Sturgeons project:

It has been reported that catches of Beluga Sturgeon (Huso huso) and Russian Sturgeon (Acipenser gueldenstaedti) reached a last peak following completion of Iron Gates I due to many migrating sturgeons being trapped below the dam. In the period 1972-1976, 115.7 t of Beluga and Russian Sturgeon were caught below the dam, representing an almost 25% increase over the five-year period prior to dam construction. However, a sharp decline followed directly after 1976, dropping to only 37.3 t for the period 1980-1984, when Iron Gates II was built, and stocks have continued to fall since.”

The decline was such that the species’ current conservation status is “Critically Endangered” From 1987 to 2007, the numbers have plummeted by 90 percent – and the decline seems continuous and steep. Additional contributing threats include overfishing, pollution, and other forms of habitat loss. 

A dam on a river in Romania.
Iron Gate hydroelectric plant on the Danube River, Romania. Photo: CIA World Factbook, public domain.

As the Iron Gates dams are too strategically important to ever remove, today, there are initiatives by the Commission for the Protection of the Danube River (ICPDR), the EU Strategy for the Danube Region (EUSDR), and other partners to add the missing infrastructure and make the Iron Gates dams passable to sturgeons. However, this would require ambitious action of all stakeholders – and a whole lot of political will which doesn’t seem to be there.

The Pan-European Atlas of In-Stream Barriers

Attempts to realistically quantify the connectivity European rivers had been delayed by the lack of a unified barrier database – an issue that demanded urgent attention.

In 2020, European researchers gathered around the project called Adaptive Management of Barriers in European Rivers (AMBERand issued a much-needed, profoundly important report – the Pan-European Atlas of In-Stream Barriers

A screenshot of an online map showing the location of dams in Europe.
Screenshot from the Pan-European Atlas of In-Stream Barriers, acquired 30 January, 2023.

It was a first-to-date extensive mapping attempt of the existing barriers on European rivers – not only the hydroelectric dams but also a prevailing number of much lower structures such as weirs, culverts, fords, and ramps. 

The team also published a complementary study in Nature.

Initially, this assessment compiled information on the already-cataloged 630,000 barriers. However, fieldwork revealed an unpleasant surprise. 

The AMBER researchers did a walkover survey of 2,715 kilometers of stream length along 147 European rivers. They found around 400,000 previously unknown barriers – meaning that more than one-third of all barriers they encountered were uncataloged.

Their final integrative dataset numbered over 1 million river barriers on European rivers, making Europe the most obstructed river landscape in the world.

The Main Findings of the Study

The final report utilized the record and field data from 36 European countries. Here are its main revelations.

  • Europe’s rivers have at least 1.2 million transverse structures within their respective courses, with a mean density of 0.74 barriers per kilometer.
  • Previous records underestimated barrier numbers by about 61 percent.
  • The barriers include large dams, but the biggest share belongs to low-head structures – 68% of documented barriers were less than two meters in height. These include culverts, fords, sluices, ramps, and weirs.
  • At least 150,000 of the barriers are old (some more than a century) and obsolete, with no economic purpose.
  • Some regions that were previously considered to have good river connectivity were found to be also congested by barriers. In the Balkans, the field validation indicated that 80% of found barriers were not registered in older inventories, making the fragmentation of these rivers much worse than previously thought.
  • The highest barrier densities were found in the heavily modified central European rivers; the regions with the lowest barrier densities were the remote and sparsely inhabited mountainous areas.
  • The main predictors of barrier density were agricultural activity in the region, the number of river-road crossings, the area of surface water, and elevation.
  • Relatively unfragmented rivers are still found in several regions: the Balkans, the Baltic states, and parts of Scandinavia and southern Europe; however, they are in grave need of protection from the intended new dam developments.

The study undoubtedly points out that the European rivers are alarmingly and increasingly fragmented, with low connectivity sure to negatively affect already-declining biodiversity and fish stock in the future.

The Future of European Rivers – Obstructed or Flowing?

The silver lining of the AMBER study is that it foresees opportunities for reconnecting European streams and rivers. Hopefully, it has already established some impact on the European Union decision-makers. Now that the physical barriers have been well-charted, they have become much easier to handle.

Our results feed directly into the new EU Biodiversity Strategy and will help to reconnect at least 25,000 km of Europe’s rivers by 2030“, said Carlos de Garcia de Leaniz, the professor at Swansea University leading the study and the project coordinator.

As a part of the project, obsolete weirs were removed in the United Kingdom, Ireland, Spain, and Denmark. Denmark has already put in the effort to reconnect 310 kilometers of its rivers.

A stone dam with three bridges in the middle of a river in England with grass on the banks.
The River Nidd spills over the 71 mete high stone dam of Scar House Reservoir in Nidderdale, Yorkshire, England, UK, built between 1921 and 1936. Photo: © Alan/stock.adobe.com.

In 2021, a record number of European river barriers were removed during a single year – at least 239 of them – from dams to weirs across 17 countries. Spain was the most enthusiastic on the list, taking out 108 redundant barriers. Perhaps it is only a tiny dent in the total of 1.2 million structures – but it’s a start.

Not all barriers have to be removed – some can stay with certain modifications applied. 

Dr. Martin Pusch, an expert in river basin management from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) and the study’s co-author, explains both the logic behind the barrier modification and its broader significance.

The AMBER Barrier Atlas gives us the opportunity to reverse the fragmentation of rivers all over Europe; to rehabilitate many of these barriers through simple technical measures, or to remove them completely through renaturation measures. Locks and large hydropower plants should be equipped with functioning fish passes for both directions of migration, and small hydropower plants, which hardly contribute to the energy transition, should be dismantled.”

He also reflects on the possibilities of fish population recovery and climate mitigation on the example of Germany – a country with a mean density of two barriers per kilometer of river stream.

“In this way, the eel population in Germany could be protected, and salmon and sturgeon could also permanently colonize our streams and rivers. At the same time, these freshwaters would be made fit for climate change and greatly enhanced as recreational areas.” – said Dr. Pusch.

The results also considerably help a growing number of activists fighting for the restoration and rewilding of rivers. Many prominent conservation non-profits have partnered around the Dam Removal Europe initiative; in 2022, DRE premiered the film Dambusters, documenting dam removal efforts in five European countries. 

River Restoration vs. New Hydropower Projects

Although things are moving forward regarding the redundant barriers, the hydroelectric power plant surge is a growing concern. The Europe’s hunger for “green kilowatts,” propelled by the need to meet the countries’ individual and collective carbon neutrality goals, is fueling the investments into questionable or outright damaging projects.

In 2019, WWF and its partners published a report on the planned hydropower projects on European soil. 

Their key points are that, despite being already loaded with the existing 21,387 hydropower plants, European rivers can expect:

  • 8,785 additional plants – planned or under construction.
  • 28 percent of these power plants are in protected natural areas (33% in the EU).
  • 91 percent of the cataloged plants are small-scale producers of less than 10MW, which will deliver almost negligible amounts of energy (at a presumably high environmental cost).

Examples from Southeastern Europe already show how wrong the unselective quest for carbon neutrality can go – and the AMBER research adds to the picture. 

The IGB Ph.D. student Helena Hudek was one of the researchers involved in the field mapping of barriers. 

She shared that the team was “shocked to see that, for example, many streams in south-eastern Europe were dry because the water had been diverted through pressure tunnels to a hydroelectric power station. In other streams, we found almost no fish and small animals because they were regularly washed away by water surges from a hydroelectric power plant.

Helena’s observations are in line with many local reports on the destruction caused by the so-called Balkan dam boom – promoted by the web of government officials, investors, and international bank capital, pushing for small-scale hydropower projects with an unclear or negative impact on the environment and communities, and suffering from poor on-site management.

In Conclusion – A Paradigm Shift Is Required

We now know how waterway obstructions can affect the rivers’ delicate ecological balance. These pivotally important ecosystems, already burdened by climate change, continue to suffer devastating losses from the effects of in-stream barriers – many of which are unnecessary.

Although large hydropower projects were seen as a main threat to river biodiversity until recently, with notable negative examples such as the Danube’s Beluga Sturgeon’s demise, the danger of small barriers is just starting to become recognized. 

Europe is on the right track with its EU Biodiversity Strategy, which includes an ambitious plan to reconnect its rivers, but, as the AMBER study concludes, “achieving this will require a paradigm shift in river restoration that recognizes the widespread impacts caused by small barriers.” 

Of course, the said shift will also have to overturn the political will in favor of conservation.

A Moment For Citizen Science: The Barrier Tracker

The study was greatly helped by citizen scientist efforts to locate and report the barriers they have encountered through a phone app.

If you are residing in or just visiting Europe, you can use the Barrier Tracker, a free app created by AMBER. The app uses georeferenced data and photos provided by the users to add to the existing map of Europe’s transverse river structures, helping establish priorities for future removal or modification of the barriers. So, with minimal effort, you can help the hard-working researchers in case they missed anything.


The Studies

Belletti, B., Garcia de Leaniz, C., Jones, J. et al. (2020) More than one million barriers fragment Europe’s rivers. Nature 588, 436–441https://doi.org/10.1038/s41586-020-3005-2

Deemer, B R., Harrison, J.A., et al. (2016), Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis, BioScience, Volume 66, Issue 11, 1 November 2016, Pages 949–964, https://doi.org/10.1093/biosci/biw117

Graham, L.J., Murphy, B.R. (2007) The Decline of the Beluga Sturgeon: A Case Study about Fisheries Management. Journal Of Natural Resources & Life Sciences Education Volume 36 2007 https://awsassets.panda.org/downloads/5_2007_decline_of_beluga_sturgeon_graham_et__al_usa_1.pdf

Siyue Li, Y. Jun Xu, Maofei Ni (2021) Changes in sediment, nutrients and major ions in the world largest reservoir: Effects of damming and reservoir operation, Journal of Cleaner Production, Vol. 318, 128601, ISSN 0959-6526, https://doi.org/10.1016/j.jclepro.2021.128601.

Articles and Other Resources

AMBER Consortium (2020). The AMBER Barrier Atlas. A Pan-European database of artificial instream barriers. Version 1.0. June 29th 2020. https://amber.international/european-barrier-atlas/

Beluga Sturgeon. Life For Danube Sturgeons.

Ecological status of surface waters in Europe. European Environmental Agency. 18 November 2021 https://www.eea.europa.eu/ims/ecological-status-of-surface-waters

Europe’s rivers fragmented by one million barriers. Nadja Neumann, IGB Press Release. 17 December 2020 https://www.igb-berlin.de/en/news/europes-rivers-fragmented-1-million-barriers

How Dams Damage Rivers. American Rivers

Hydropower pressure on European rivers: The story in numbers. WWF. 2 December 2019https://www.wwf.eu/wwf_news/publications/?uNewsID=356638

More Than 1 Million Barriers Destroying Europe’s Rivers, New Research Shows. WWF. 17 July 2020. https://wwf.panda.org/wwf_news/?364617/1-million-dams

Record number of dams removed from Europe’s rivers in 2021. Graeme Green, The Guardian. 16 May 2022 https://www.theguardian.com/environment/2022/may/16/record-number-of-dams-removed-from-europe-rivers-in-2021-aoe

The Iron Gates Dams & Disruption of Spawning Migration. Life For Danube Sturgeons.


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About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.